Question: On the first 4 tests of his math class, Michael got an average score of 88. What does he need on the next test to have an overall average of 90?
Explanation: Let his score on the next test be $x$ The sum of all of his scores is then $4 \cdot 88 + x$ The same sum must also be equal to $5 \cdot 90$ Solve: $x = 5 \cdot 90 - 4 \cdot 88 = 98$.